Vehicle wheel speed sensors can be divided into two broad categories, active and passive. Active sensors continually provide outside power to a sensor component, such as a hall effect transducer, and are consequently relatively expensive. Passive sensors, since they need no outside power, are relatively less expensive. Passive sensors provide a magnetic field with some type of magnet, generally combined in proximity with a sensing coil into a single sensor component. The coil-magnet component is usually mounted fixed relative to the vehicle, since it has signal wires leaving it. A variable reluctance ring, generally a toothed iron ring, is mounted so as to turn at the speed of the wheel, within the magnetic field, thereby creating an electric signal that varies with wheel speed.
Passive sensors are often combined with a bearing assembly, either directly with the wheel bearing, or with a bearing somewhere else in the drive line. This is an obvious choice, since a bearing provides a fixed race and a race rotating at wheel speed in close proximity. The ring can be mounted to the rotating race and the sensor component to the fixed race. Combining such a sensor with a bearing provides another advantage in that bearings typically already have a seal that encloses an annular space between the races, protecting the bearing rolling elements and retaining lubricant. If the annular space is large enough, it is a simple matter to mount the ring to the rotating race within the annular space, where it is shielded by the bearing seal that is already there. The same can be done for the sensor component, depending on what type it is.
The coil-magnet components in passive sensors fall into two broad sub categories, the probe type and the circumferential type. In the probe type, a coil wraps a magnet and iron pole piece, in a general probe shape. The probe can be inserted either through a hole in a bearing housing, or, alternately, through a hole directly in the bearing seal, depending on the room available and on how solidly mounted the seal is. The end of the probe then faces the ring, and both the end of the probe and the ring are protected within the enclosed annular space. In the circumferential type, an annular magnet and wire coil are combined into a ring-shaped component. This single component, if small enough, can be mounted to the fixed race within the annular space, near the rotating toothed ring, and both protected by the same seal. In either case, removing the coil-magnet component for servicing involves either leaving an empty hole through the seal, in the case of the probe type sensor, or removing the entire seal, in the case of the circumferential type sensor. This leaves the bearing and its lubricant supply exposed. This problem could be avoided by simply plugging the holes during the servicing process. Or, the bearing could be redesigned with a separate sealed chamber for the sensor components entirely outboard of the bearing seal. Either alternative involves extra steps and expense, however, and the latter could potentially take up significantly more space.